Noise problems soundproofing new build semi-detached house?

That's how it is. He'll just rent to retirees next, where hearing might already be a bit weaker.

Well, the other tenants are not bothered by it

If the mistake was only made on their half and not on the TE's, that is logical and has nothing to do with different personal sensitivity at all.

He’ll just rent to retirees next, whose hearing might already be a bit weak.

Deafness of the ears is of no use against vibrations.

1.4mm joints are just too little in my opinion, normally they are more like 2cm and up. You can already see in the picture how the mortar between the bricks is touching. That's definitely not normal, I also have direct neighbors with separate walls, and you can't hear anything except the staircase mentioned here at night. There was definitely shoddy work done on your side. If I were you, I would close the chapter with the house and move out; arguing with the landlord also makes little sense in my opinion, the hassle is greater than the possible gain.